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<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .a {
            width: 25%;
            height: 200px;
            border: 1px solid #000;
            float: left;

            /* 把标准盒模型转变为怪异盒模型 --- 内边距和边框不再把盒子撑大 */
            /* border-box 怪异     content-box 标准 */
            box-sizing: border-box / content-box;
        }
    </style>
</head>

<body>


    <div class="a"></div>
    <div class="a"></div>
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</body>

</html>